Protective headgear have impact resistant structural shells. These structural shells each have a critical load threshold. An incremental increase in load above the critical load threshold results in a buckling of the structural shell. This buckling decelerates the force of a blow over a time interval, thus decreasing the impact energy of the blow. The critical load threshold relates primarily to the structure of the structural shell, as the critical load threshold is, typically, below the yield or fracture strength of the material. The ability of protective headgear to withstand a given impact without buckling is, of course, dependent upon the critical load threshold of the structural shell. Corrugations have been used to increase the critical load threshold of the structural shell. The structural shells have been used in combination with foam inserts, which crush under impact conditions to further decrease the impact energy of the blow.